Ultrasonic flow meters are often used in process and automation technology. They allow determination of the volumetric flow rate and/or mass flow in a pipeline in a simple way.
Known ultrasonic flow meters often work according to the runtime difference principle. In the runtime difference principle, the different run times of ultrasonic waves, in particular ultrasonic pulses, so-called bursts, are evaluated relative to the direction of flow of the liquid. To this end, ultrasonic pulses are transmitted at a certain angle to the pipe axis, both in and against the direction of flow. Using the runtime difference, the flow rate and thus, the volumetric flow rate can be determined if the diameter of the pipeline section is known.
The ultrasonic waves are generated or received by means of so-called ultrasonic transducers. For this purpose, ultrasonic transducers are firmly attached to the pipe wall of the relevant pipeline section. These are also available as clamp-on ultrasonic flow measurement systems. In these systems, the ultrasonic transducer are pressed from outside of the measurement pipe against the pipe wall. A big advantage of clamp-on ultrasonic flow measurement systems is that they do not contact the measurement medium and are mounted on an existing pipeline.
The ultrasonic transducers usually consist of an electromechanical transducer element, e.g. a piezoelectric element, and a coupling layer. In electromechanical transducer element, the ultrasonic waves are generated as audible signals, and are guided to the pipe wall via the coupling layer, and from there, led into the liquid in case of clamp-on systems, or they are coupled to the measurement medium via the coupling layer in case of inline systems. Then, the coupling layer is also rarely called membrane.
An additional coupling layer, a so-called adaptation layer can be arranged between the piezoelectric element and the coupling layer. The adaptation layer takes on the function of the transmission of the ultrasonic signal and simultaneous reduction of reflection caused by different acoustic impedances at boundary layers between two materials.
Both in the clamp-on systems as well as the inline systems, the ultrasonic transducers are arranged in a common plane on the measurement pipe, either on opposite sides of the measurement pipe, in which case the acoustic signal traverses the measurement pipe once along a secant, projected onto a pipe cross-section, or on the same side of the measurement pipe, in which case the acoustic signal is reflected at the opposite side of the measurement pipe, whereby the acoustic signal traverses the measurement pipe twice along the secant projected onto the measurement pipe. U.S. Pat. Nos. 4,103,551 and 4,610,167 show ultrasonic flow meters with reflections at reflection surfaces provided in the measurement pipe. Multi-path systems, which comprise a plurality of pairs of ultrasonic transducers, each of which form a signal path, along which the acoustic signals pass through the measurement pipe have also become known by now. The respective signal paths and the associated ultrasonic transducers are in mutually parallel planes that are also parallel to the measurement pipe axis. U.S. Pat. Nos. 4,024,760 or 7,706,986 are examples of such multi-path systems. An advantage of multi-path systems is that they measure the profile of the flow of the measurement medium in the measurement pipe at several points, thus being able to provide highly accurate readings for the flow. This is achieved, among other things, by the fact that the individual run times along the different signal paths are weighted differently. However, a disadvantage of multi-path systems is that their manufacturing costs are high, due to the installation of a plurality of ultrasonic transducers and, where appropriate, complex evaluation electronics.
There are various papers for weighting of the signal paths. The “Comparison of integration methods for multipath acoustic discharge measurements” paper by T. Tresch, T. Staubli and P. Gruber in the supplementary publication for 6th International Conference on Innovation in Hydraulic Efficiency Measurements, Jul. 30-Aug. 1, 2006 in Portland, Oregon, United States, compares current methods for weighting the run times along different signal paths to calculate the flow rate.
European Patent, EP 0 715 155 A1 comprises a measurement arrangement with multiple refraction, wherein the subsections of the signal path only constitute a plane that is parallel to the measurement pipe axis. The reflection surfaces on which a first subsection of the signal path ends and a second subsection of the signal path connects are shown in EP 0 715 155 A1 as flat moldings that are attached to the inside of the pipe. Although it is theoretically possible to introduce reflection surfaces from the end faces of a measurement pipe and then weld them in place on the inner wall of the measurement pipe, such a production quickly reaches its limits for smaller measurement pipes with small nominal diameters, as a welding device can be used at great expense and with loss of precision in the positioning of the reflection moldings in case of small nominal diameters. Thus, the theory of EP 0 715 155 A1 is applicable to measurement pipes with large nominal diameters.
German Patent, DE 10 2008 055 030 A1 describes a connector molded by hydroforming in an ultrasonic flow meter. An ultrasonic transducer is inserted in this connector. The signal is transmitted along a straight signal path without the signal being reflected at the pipe wall. The measurement pipe of the flow meter in this case has a flat shape, so that less flows can occur in the flow profile in this pipe through turbulence, unlike round cross sections.
German Patent, DE 102 49 542 A1 describes a coupling surface for coupling an ultrasonic signal from an ultrasonic transducer to a measurement pipe, wherein the coupling surface formed from the measurement pipe has an oblique shape. The measurement pipe also has a molding 10, which provides a reflection surface.
European Patent, EP 0 303 255 A1 describes a measurement pipe of an ultrasonic flow meter, in which a reflection surface is formed integrally with the measurement pipe. Over a wide range, this leads to an average expansion of the measurement pipe, which is unfavorable for the accurate determination of the measured data.
In contrast, German Patent, DE 10 2012 013 916 A1 as well as FIGS. 6 and 7 of the present application shows a measurement pipe of an ultrasonic flow meter with screw-fitted reflection surfaces. First, a connector provided with a thread is formed, in which a reflector can be inserted subsequently. This production version has proven itself, in principle, for all measurement pipes, regardless of their nominal diameter. However, the production requires strict compliance with specified drill patterns and each connector must be machined separately before insertion of the reflector.
An alternative already known variant is the casting of the pipe and the welding of nozzles on the measurement pipe and the subsequent screwing or welding on a reflective surface.